Throw a less cryptic error for a missing twitter:image attribute
Throw a less cryptic error for a missing twitter:image attribute

Previously this gave the less-than-specific output:

    Checking Twitter card metadata...
    jekyll 4.2.2 | Error:  bad argument (expected URI object or URI string)

which is because the value being passed to the `URI` is `nil`.

alexwlchan/alexwlchan.net,alexwlchan/alexwlchan.net,alexwlchan/alexwlchan.net,alexwlchan/alexwlchan.net,alexwlchan/alexwlchan.net